Crackle/pop from right speakers

[gsmiller]

The front right and right door speakers on my 02 Boxster S don't work, but manage to put out a nice crackle/pop. The car has the premium Bose sound package with stock head unit. If I adjust the balance all the way left, everything sounds fine. Also, if I adjust the fader all the way to the rear, I can adjust the balance all the way left or right and it sounds fine. If any signal is going to the front right speakers, I just get a steady low hiss and some popping. This pop happens every time I adjust the volume or balance/faders.

From looking at the wiring diagrams, I can't imagine there is a single common failure point for both the front right and side right speakers. I pulled the head out and the plugs looked OK. The connections to the amp also look fine to my untrained eye. Any guesses as to what it could be? Likely a faulty head unit? Faulty amp? Any way to test before going down the route of getting a new head unit or new amp? Thanks!